1. Field of the Invention
The present invention relates to a capacitor structure, and more particularly, to a capacitor structure applied to a thin film transistor liquid crystal display (TFT-LCD) to improve contrast of the display by increasing capacitance value and aperture ratio.
2. Description of the Prior Art
Currently, liquid crystal displays(LCDs)area mature flat panel display technology. Applications for liquid crystal displays are extensive and include mobile phones, digital cameras, video cameras, notebooks, and monitors. Due to high quality display requirements and the expansion of new application fields, the LCD has developed toward high quality, high resolution, high brightness, and low price. A low temperature polysilicon thin film transistor liquid crystal display (LTPS-TFTLCD) is one type of thin film transistor liquid crystal display. The low temperature polysilicon thin film transistor liquid crystal display, being actively driven, is a break-through in achieving the above objectives. Furthermore, the metal-oxide-semiconductors and the low temperature polysilicon thin film transistorsused in this technique are integrated in the same manufacturing process to fabricate a system on panel (SOP). Therefore, technological innovation based on this concept has become an important subject for further development.
A thin film transistor liquid crystal display (TFT-LCD) utilizes thin film transistors (TFTs) as switches of active matrix to control the charges of pixel electrodes. By rotating the liquid crystal molecules of the liquid crystal unit in the pixel to expected angles, the amount of light passing through the pixel is controlled. Referring to FIG. 1, FIG. 1 is an equivalent circuit diagram of a pixel 20 in a TFT-LCD. As shown in FIG. 1, the pixel 20 comprises a liquid crystal unit (LC) electrically connected to a common counter electrode (CE) and a thin film transistor 22. A gate electrode 24 of the thin film transistor 22 is electrically connected to a scan line G0, a drain electrode 26 of the thin film transistor 22 is electrically connected to a signal line D0, and a source electrode 28 of the thin film transistor 22 is electrically connected to a pixel electrode (not shown) of the LC. The pixel 20 further comprises a storage capacitor (SC) electrically connected between the LC and a scan line G1. Since the capacitor can increase or decrease the amount of charge by charging or discharging depending on the variation of voltage, the storage capacitor SC is not only used to reduce the voltage variation of the LC due to leakage current, but also is used to assist the LC with storing electric charges. The turned-on time for the pixel 20 is thus lengthened.
Referring to FIG. 2, FIG. 2 is a structural schematic diagram of a thin film transistor liquid crystal display 30 according to the prior art. As shown in FIG. 2, the prior art thin film transistor liquid crystal display 30 comprises a substrate 32. The substrate 32, composed of transparent materials, is an insulating substrate, and the substrate 32 comprises a glass substrate, a quartz substrate, or a plastic substrate. A pixel array area 33 and a periphery circuit area 34 are included on a surface of the substrate 32. A thin film transistor area (TFT area) 35, a capacitor area 36, and an aperture region 37 are included in the pixel array area 33. The thin film transistor area 35 is used for disposing a thin film transistor 38, and the capacitor area 36 is used for disposing a storage capacitor 42. Since a gate 44 of the thin film transistor 38 is composed of a low temperature polysilicon material, the thin film transistor 38 is a low temperature polysilicon thin film transistor.
The storage capacitor 42 comprises a first insulating layer 46 disposed on the substrate 32, a first metal layer 48 disposed on the first insulating layer 46, a second insulating layer 52 disposed on the first metal layer 48, a second metal layer 54 disposed on the second insulating layer 52, a third insulating layer 56 disposed on the second metal layer 54, and an organic coating 58 disposed on the third insulating layer 56. A contact hole 62 is included in the organic layer 58 and the third insulating layer 56 to expose portions of the second metal layer 54. A transparent conductive layer 64, extending into the contact hole 62, is disposed on the organic layer 58 and connected to the second metal layer 54.
Both the first insulating layer 46 and the second insulating layer 52 are silicon oxide layers. The first metal layer 48 and the second metal layer 54 may each be a tungsten layer (W layer) or a chrome layer (Cr layer). The third insulating layer 56 is a silicon nitride layer. The organic layer 58, formed by a spin coating method, is an organic material layer. The transparent conductive layer 64 comprises an indium tin oxide layer (ITO layer) or an indium zinc oxide layer (IZO layer). The first metal layer 48 is used as the bottom electrode plate of the storage capacitor 42, the second metal layer 54 is used as the top electrode plate of the storage capacitor 42, and the second insulating layer 52 is used as the capacitor dielectric layer of the storage capacitor 42.
However, the prior art capacitor 42 applied to the thin film transistor liquid crystal display 30 is essential barring any break though. The capacitance value of a capacitor directly reflects its charge storage ability. The larger the area of the electrode plate is, the higher the capacitance value is. In other words, the number of charges stored by the capacitor is proportional to the area of the electrode plate. Although the capacitor has a large area electrode plate and can store more charges to lengthen the turned-on time for the pixel, a large area is occupied by the capacitor to reduce the aperture ratio. Therefore, not only is the maximum brightness of the liquid crystal display limited, the overall contrast is reduced. Under the limitation of insufficient aperture ratio in the pixel, special processing or consideration is required when designing or producing products to result in raised cost, leading to uncompetitive products.
Therefore, it is very important to develop a new capacitor structure having a small area to store a specific number of charges so as to improve the contrast of the liquid crystal display when applied to the thin film transistor liquid crystal display. Furthermore, current processing should not need to be changed when fabricating the capacitor structure to produce competitive products.